It may be useful to determine a location and/or a distance of a remote wireless beacon. More particularly, it may be useful to use the beacon to determine a location of an object or person, such as a firefighter within a building, for example. Beacons may also be deployed to enhance navigational capabilities, such as providing 3D navigation for ships within ports.
A remote wireless beacon may be used in determining the location of the remote object or person. A remote beacon generally receives a transmitted signal from a remote transmitter and processes and transmits a return signal based upon the transmitted signal. For example, the return signal of a typical location beacon may provide timing information to assist in determining a range to the remote device. Alternatively, a GPS-determined location may be included in the return signal from a conventional navigation beacon. In accordance with the present invention, the beacon described herein generates the return signal by mixing the incoming signal with the local oscillator signal without requiring any decoding of the input signal. This configuration provides the lowest latency possible and minimizes return delay.
U.S. Patent Application Publication No. 2008/0070532 to Moffatt et al. and assigned to assignee of the present application, Harris Corporation of Melbourne, Fla., and incorporated herein by reference, discloses actively determining the range of (i.e. distance to) a remotely located radio transmitter, receiver, or radio transceiver. The method includes monitoring RF emissions of the RF receiver, generating an RF signal on an RF frequency that can be received by the RF receiver, and detecting a variation in the receiver RF emission or emissions responsive to the RF signal. The variation in the RF emission includes small changes in one or more of the amplitude, phase, or frequency of one or more local oscillator signals, harmonics thereof, or mixing products in the receiver. These changes are caused by the transmitted RF signal's influence on the receiver's circuitry. The transmitted RF signal can be used to cause a phase, frequency, amplitude, or combination of phase, frequency, or amplitude modulation of certain signals in the receiver. Such modulation can be caused by various aspects of the receiver design, such as intended or unintended coupling that exists between the local oscillator and other components of the receiver.
Other ranging methods, for example, a ranging algorithm, may be used to compute a distance between two radio transceivers. A basic ranging algorithm may compute correlation in the time domain between a transmitted signal and a received signal. A peak of such correlation may be related to the round trip time delay, often specified in samples. If the delay is some fraction of a sample, the peak may be smeared in the correlation. This may be greatly affected by noise and multipath, for example. A typical ranging technique may include a curve fit, neighbor weighting, or another technique to attempt to resolve delays.
U.S. patent application Ser. No. 12/629,584 to Hoffmann et al. and assigned to assignee of the present application, Harris Corporation of Melbourne, Fla., and incorporated herein by reference, discloses a multi-carrier waveform for actively determining a fine range estimate of a remotely located wireless device. This multi-carrier waveform may be used to improve time resolution and range accuracy. The RF interrogator may be configured to generate the multi-carrier base waveform as a multi-carrier orthogonal frequency division multiplexed (OFDM) base waveform, for example. The multi-carrier waveform may be constructed with a low peak-to-average power ratio of about 2.6 dB, regardless of the number of subcarriers, for efficient transmission using a power amplifier. The multi-carrier waveform may be transmitted at the first RF frequency to the remote wireless device, such as a beacon device. The signal processing engine may also process the frequency domain data by at least determining a channel estimate of the return waveform. The channel estimate may be based upon a mathematical inversion of the ratio of the received-to-transmitted sounder waveform, for example. The signal processing engine may also process the frequency domain data by at least resolving frequencies in a channel estimate of the frequency domain data, and resolve frequencies in the channel estimate based upon a singular value decomposition (SVD), for example. Frequency offset may result from Doppler due to a mobile RF interrogator, mobile wireless beacon, or both. Frequency offset may also occur from the LO frequency at the wireless beacon. The RF interrogator may also process the frequency domain data by at least reducing noise in the channel estimate and removing frequency offset. Thus, the multi-carrier waveform may be used to obtain an accurate round-trip time from the RF interrogator to the wireless beacon in order to estimate an accurate distance, referred to as fine range resolution.
Further, by incorporating multiple receiver and/or transmit antennas in the beacon, a Multiple-Input/Multiple-Output implementation (MIMO) may be used to provide spatial-diversity and discriminate different beacons coexisting on the same frequency and to improve location accuracy and/or data capacity.
There is a persistent need for a low complexity beacon having low cost and ease of production to provide location detection and/or tracking of disadvantaged nodes, i.e. items that do not already include a radio transmitter, receiver, or transceiver. Such a beacon should be low Size, Weight, and Power (SWAP).